1. Field of the Invention
The present invention relates generally to artificial lens implants for the human eye, and is particularly concerned with an intraocular lens for implantation in the posterior chamber of the eye following extracapsular cataract extraction.
2. Description of the Prior Art
A cataract is a condition in which the natural crystalline lens of the eye becomes partially or completely opaque. This can be a result of trauma, disease or of the natural aging process. Congenital cataracts also occur in some cases. Whatever the cause, the condition usually worsens over time, eventually reaching a point where vision is severely impaired. Special eyeglasses can be used to restore vision to some patients, but in many cases the cataract condition progresses to a point where surgery becomes necessary.
Cataract surgery involves the removal of the clouded natural lens, in whole or in part, followed by the substitution of some type of artificial lens. The artificial lens insures that light images are properly focussed on the retina in the absence of the natural lens. Before the development of implanted lenses, the artificial lens usually consisted of a corneal contact lens or aphakic eyeglasses. With aphakic eyeglasses, however, many patients experience undesirable side effects such as double vision, reduced side vision, and distortion of objects. Contact lenses eliminate some of these problems, but are difficult to handle and cannot be physically tolerated by some patients, particularly elderly patients. An additional problem with any type of external lens is, of course, that the patient is effectively left with no vision in the affected eye when the lens is removed.
This situation was greatly improved with the advent of the artificial intraocular lens (IOL). This type of lens is permanently implanted in the eye after cataract surgery to replace the natural crystalline lens. The implanted lens usually occupies a position within the eye that is relatively close to the position formerly occupied by the natural lens, and therefore serves as a nearly physiologic substitute with virtually no undesirable side effects on vision. A further advantage of this type of lens is that the patient is not faced with the difficulties involved in handling an external lens, or with the complete loss of vision that occurs when an external lens is removed. Of course, since the implanted IOL does not have the ability to focus by changing shape, as can the natural lens, patients receiving artificial lens implants may require eyeglasses for reading or distance. However, from the standpoint of a patient who has previously had a cataract, the occasional need for eyeglasses is usually considered to be a minor inconvenience.
Prior to the implantation of an artificial intraocular lens, a surgical procedure must be performed to remove the natural lens which contains the cataract. This may be done in one of two ways. In the procedure known as intracapsular cataract extraction (ICCE), the entire lens is removed including the surrounding capsule. The second procedure, known as extracapsular cataract extraction (ECCE), involves opening or excising the anterior capsule, and then removing the lens substance from the interior of the capsule. This is usually done in a piecemeal manner using either an ultrasonic phacoemulsification instrument or, more commonly, an irrigation-aspiration technique. The ECCE procedure leaves the posterior capsule in place, together with the zonular ligaments which join this membrane to the ciliary body. Generally, a portion of the anterior capsule in the form of an irregular annular flap is also left in place after ECCE. The annular cavity between the anterior capsule flap and the posterior capsule is referred to as the cleft or fornix of the capsule.
According to Dr. John H. Sheets, in the text entitled Covered Bridge: An Update on Lens Implantation (1977), the development of the intraocular lens can be traced back to 1949. Dr. Harold Ridley of Great Britain recognized that wartime pilots who had suffered eye injuries from the shattered plastic windshields of their aircraft had no adverse reaction to the plastic fragments embedded within the eye. This led Dr. Ridley to develop an artificial intraocular lens, made of a clear plastic material, that could be implanted in the posterior chamber of the eye following extracapsular cataract extraction. Unfortunately, implantation of the Ridley lens was followed by post-operative complications in a relatively high percentage of cases, sometimes requiring later removal of the implanted lens. Some of these problems were believed to be due to the size and weight of the lens, which were very great by modern standards. Post-operative clouding of the posterior capsule was also observed after implantation of the Ridley lens, a problem which still persists at the present time.
In part because of the problems experienced with early posterior chamber lens designs, many subsequent designs provided for implantation of the lens in the anterior chamber of the eye. These lenses were usually provided with peripheral supporting structures which could be placed within the angle at the intersection of the iris and the cornea. By virtue of their placement in the anterior chamber, which is more readily accessible to the surgeon, these lenses were somewhat easier to implant than the earlier posterior chamber lenses. Unfortunately, the angle fixation method used in most anterior chamber lens designs sometimes gave rise to problems such as iritis, glaucoma and corneal dystrophy.
The next step in the evolution of intraocular lens implants was the development of the iris fixation lens. This type of lens was secured to the iris by means of loops, clips, sutures or a combination of these. Iris fixation lenses were positioned either in the anterior chamber, in which case the lens was referred to as a pre-pupillary lens, or in the plane of the iris, in which case the lens was referred to as a pupillary or iris plane lens. In addition to avoiding many of the problems associated with angle fixation lenses, iris fixation lenses had the further advantage that the fixation method inherently resulted in the lens being properly centered with respect to the pupil. Iris fixation lenses, however, were not without their own problems. Principal among these were occasional subluxation or dislocation of the implanted lens, and possible interference with normal dilation and contraction of the iris.
Although many types of anterior chamber and iris fixation lenses are in use at the present time, there is an increasing preference for posterior chamber implantation as originally envisioned by Dr. Ridley. The posterior chamber is the location of the natural crystalline lens before its removal, and for that reason, it is generally considered to be the most appropriate location for a truly physiologic lens implant. A typical present-day posterior chamber lens design, based on a much earlier anterior chamber lens developed by Barraquer, consists of a lens body supported by at least two curved strands or loops of resilient material extending outwardly from the edge of the lens body. U.S. Pat. No. 4,159,546, to Steven P. Shearing, describes a lens of this type and a method for implanting the lens in the posterior chamber following intracapsular or extracapsular cataract extraction. The implantation method involves placing the curved ends of the resilient loops against the ciliary body in the groove or sulcus immediately behind the iris. The spring-like resilience of the loops provides proper centering and fixation of the lens within the posterior chamber. In cases where the lens is implanted following extracapsular extraction (ECCE), the posterior lens surface can be further supported by the posterior capsule to achieve capsular fixation. Currently available versions of the Shearing lens include one type in which the resilient loops are made slightly shorter to allow them to be placed within the cleft or fornix of the capsular remnant, rather than in the ciliary sulcus.
At the present time, ECCE is the preferred technique of cataract extraction since it leaves the posterior capsule in place and thereby maintains the physiologic barrier between the posterior chamber and the vitreous body. The maintenance of this barrier is believed to eliminate or at least reduce a number of post-operative complications, particularly cystoid maculopathy, retinal detachment, and vitritis. Unfortunately, with the posterior capsule left in place, it frequently happens that residual epithelial cells proliferate and migrate from the equatorial region of the capsule toward the center, causing gradual clouding of the capsule and eventually impairing vision. The continued proliferation of these live cells results in the formation of cellular deposits which are known as Elschnig's pearls. Fibrous scar tissue also forms on the posterior capsule in many cases. Scraping or polishing of the posterior capsule is usually performed during the initial ECCE procedure to remove any cellular debris, but it is nearly impossible to remove all of this material.
Post-operative clouding of the posterior capsule has usually been remedied in one of two ways, both involving minor follow-up surgery. In the first method, a cutting or discission of the central posterior capsule is performed in order to allow light to pass through the clouded capsule to the retina. The second method involves repolishing or scratching the posterior capsule to eliminate the clouding condition. The problem with this approach, however, is that the clouding condition can reappear fairly rapidly unless the posterior capsule has been carefully scraped out to its peripheral edge. Regardless of which method is chosen, additional difficulties are presented when a posterior chamber lens implant is already in place, since it is then necessary to manipulate the discission knife or scratching instrument behind the lens.
There has been at least one attempt to design a posterior chamber lens implant which would eliminate clouding of the posterior capsule due to epithelial cell migration and fiber growth. U.S. Pat. No. 4,244,060, to Kenneth J. Hoffer, describes a posterior chamber lens which incorporates an annular ridge or lip projecting directly rearward from the central optical region of the lens body. The ridge or lip is intended to seat against the posterior capsule when the lens is implanted after extracapsular cataract extraction, thereby providing a barrier to inhibit the expansion of lens fibers or Elschnig's pearls into the central region of the capsule. Sections are left missing from the annular ridge to allow a discission knife to be inserted into the space behind the lens. In this way, a knife discission can be performed without dislodging the lens in the event that clouding of the posterior capsule occurs despite the barrier. A second function of the annular ridge or lip is to serve as a barrier for preventing vitreous material from coming forward into the anterior chamber after a discission has been done. Modified versions of the Hoffer lens are being sold at the present time, and, while some decrease in the discission rate has been reported for patients receiving this lens, there has been no conclusive proof that the ridge has a definite effect in lowering the need for discission.
A recent and important advance in the field of ophthalmology has been the use of the neodymium-YAG laser to treat clouding of the posterior capsule following extracapsular cataract extraction. The laser is simply focussed in the vicinity of the posterior capsule from a point outside the eye, and is then pulsed repeatedly until enough of the capsule is destroyed to remove the clouding condition and thereby improve vision. With the advent of the YAG laser, a condition that had previouly required follow-up surgery could now be remedied by means of a fairly simple and non-invasive office procedure. One problem that remained, however, was the potential for damage to an implanted posterior chamber lens during the laser capsulotomy procedure, particularly in cases where the lens was positioned very close to, or in contact with, the posterior capsule. In these situations, the repeated laser firings can cause undesirable pitting and cracking of the lens implant. One solution to this problem is to provide a spacing between the posterior capsule and the rear face of the lens, so that the YAG laser can be focussed in the vicinity of the capsule without damaging the posterior lens surface. The modified versions of the Hoffer lens, referred to previously, make use of this principle by employing the annular ridge or lip to separate the rear face of the lens from the posterior capsule.
Although the use of the YAG laser for posterior capsulotomies has greatly alleviated the problems attending extracapsular cataract extraction and posterior chamber lens implantation, the fact still remains that patients receiving lens implants after ECCE cannot be assured of a good result without accepting the possibility that follow-up procedures may be required to open the posterior capsule in the event that clouding occurs. In addition to clouding due to epithelial cell migration or fiber growth, other undesirable conditions affecting the posterior capsule may occur after the initial surgery. For example, it sometimes happens that large residual tags of anterior capsule are left after the anterior capsule is opened during ECCE. After surgery, these tags can become adherent to the posterior capsule, inducing fibrotic activity at their contact points. The annular pattern of scar tissue that often results from this condition is referred to as a Sommering ring. The fibrotic tissue can contract and distort the central posterior capsule, resulting in fine wrinkling of the posterior capsule behind the lens implant. This condition is believed to be responsible for the double and triple ghost images that some patients experience after implant surgery.
Either of the two conditions mentioned previously, capsular wrinkling or capsular clouding due to epithelial cell migration and fiber growth, can require a posterior capsulotomy which would not otherwise have been required. Currently available posterior chamber lens designs have not addressed these problems. While it is true that the YAG laser is useful in these situations, the laser capsulotomy procedure still has the disadvantage that it breaks the natural barrier between the aqueous and vitreous, and hence increases the risk of cystoid maculopathy, retinal detachment, and vitritis. In addition, the long term effects of the YAG laser on the eye are not known at this time.
Rather than relying on corrective follow-up procedures to open the posterior capsule after the onset capsular clouding or wrinkling, it would be far more desirable to prevent the occurrence of these conditions in the first place by appropriate design of the lens implant. In this way, the posterior capsule may be left intact and the patient may be spared the necessity of undergoing follow-up procedures after the initial implant surgery. This objective has unfortunately not been met by prior art designs for posterior chamber lens implants.